People in places that get a lot of snow know this: You can drive on it.

"Snow packs down. It's a little like brittle, it breaks up as you drive on it," said National Weather Service meteorologist Kurt Van Speybroeck said. "If you get even a 10th of an inch of ice on a road, it's like a skating rink."

Yes, snow-covered roads kill. But the bigger culprit in any snow storm is ice.

As cars travel across the snow, the heat from the tires melts it. When the cold weather refreezes it, the flakes compact into a thin sheet of ice.

That's exactly what happened in Atlanta two weeks ago.

It wasn't the snow. It was the thin layer of ice that turned a metropolitan area of 6 million into a parking lot.